A commonly used epitaxial growth technique for gallium arsenide semiconductors is organometallic vapor phase epitaxy (OMVPE). The uniformity of the doping of a given epitaxial layer is an important factor to control during the OMVPE process.
One reactor widely used for OMVPE processing is known as a flow flange reactor. An example of a flow flange reactor is the GS3200 manufactured by EMCORE of Somerset, N.J. In the EMCORE flow flange configuration, the flow flange of the growth chamber has three divided concentric sections, also known as gas dispersing rings, through which source gases are injected and dispersed. The three gas dispersing rings are also divided in half at the center, such that group III and group V source gases may be separately injected. Typically, the Group 3 and dopant source gases are injected into one side of the three sections and group V source gases are injected into the other side. The ratios of the input flows of the group III source gases to the three gas dispersing rings typically determine the thickness profile on the layer being epitaxially grown. Accordingly, the group III gas input flow ratios are ordinarily fixed and may not be adjusted to aid in achieving doping uniformity because the thickness profile of the layer being grown would be disturbed.
In the EMCORE configuration and other flow flange reactor configurations, dopant source gases are typically injected along with the group III source gases and in approximately the same ratio as the group III source gases. This configuration assumes that doping uniformity results from maintaining the ratios of dopant source flow the same as the input flow ratios for the group III gas sources. Often, as is the case in the EMCORE configuration, the dopant source gases are injected along with the group III source gases through a common gas control. A gas control is also known as a gas injector. A gas control may be, for example, a metering valve or a mass flow controller. The above assumption regarding doping uniformity, however, proves to be false. Doping uniformity is often affected by growth conditions such as the input flow ratios between the group V and group III gases, growth temperature, and, in the case of low doping levels, background doping levels. In current configurations, group V gas sources are injected into one side of the flow flange while group III and dopant sources are injected into the other side of the flow flange. Current reactor configurations are inadequate for achieving the desired degree of doping uniformity over a wide range of doping levels.